1. Field of the Invention
This invention relates to a waveguide filter adapted for use in communication devices of the microwave and millimeter wave ranges.
2. Description of the Prior Art
In satellite communication, an earth station is very remote from a satellite (in the case of a geostationary satellite) as far as 35,900 Km, so that a radio wave received by a receiver is very weak. Consequently, a filter for handling such a weak radio wave must exhibit only a small transmit transmission loss; thus, a waveguide filter having a high selectivity Q has been used widely as a filter of small loss. Similarly to the receiver side, a waveguide filter of small transit transmission loss has been used frequently on the transmitter side because the transmitter side handles a large power transmission and the energy lost in the course of transit transmission is converted into heat energy to thereby sometimes heat a transmitting apparatus.
In general, as shown in FIG. 7, the waveguide filter is configured so that shunt inductor plates are provided in a waveguide of rectangular in cross section to divide the inside into a plurality of compartments to thereby make each compartment function as a waveguide resonator, whereby a pass band is created. This device is called the shunt inductor type waveguide filter.
In FIG. 7, 41 indicates a waveguide, 42 through 49 indicate shunt inductor plates (hereinafter referred to simply as the inductor plates) which form induction windows, and 50 through 52 indicate waveguide resonators. One waveguide resonator 50 is formed by the inductor plates 42 and 46, inductor plates 43 and 47, and portions of the waveguide 41 extending between the inductor plates 42 and 43 and between the inductor plates 46 and 47; another waveguide resonator 51 if formed by the inductor plates 43, 44, 47, and 48 and the portions therebetween of the waveguide 41; and still another waveguide resonator 52 is formed by the inductor plates 44, 45, 48, and 49 and the portions therebetween of the waveguide 41, thereby resulting in the waveguide filter (a bandpass filter) having three stages consisting of these waveguide resonators 50, 51, and 52. The resonance center frequency and the passband width of the waveguide filter are determined by the dimensions: width and height of the waveguide 41, distance between the inductor plates, and width of the inductor plates 42 through 49 (or size of the induction windows).
The concrete configuration of the conventional waveguide filter will now be described with reference to FIGS. 8 and 9. In FIGS. 8 and 9, the same portion as that shown in FIG. 7 is identified by the same reference numeral with its duplicate explanation omitted.
In FIGS. 8 and 9, 53 through 60 indicate slots formed in the waveguide 41 in which the inductor plates 42 through 49 are to be inserted, 61 and 62 indicate flanges, and 63 through 70 indicate solder for fixing the inductor plates 42 through 49 inserted in the slots 53 through 60 to the waveguide 41. These inductor plates 42 through 49 are secured to the waveguide 41 by forming the slots 53 through 60 of given depth in the waveguide 41 at given intervals, inserting the inductor plates 42 through 49 larger than both the height of the waveguide 41 and the depth of the slots 53 through 60 into these slots 53 through 60, and soldering at spots 63 through 70 from the exterior of the waveguide 41. In the foregoing steps, the metal welding or like process may be adopted as the fixing step other than the soldering process, and other adequate processes may be selected. The flanges 61 and 62 are provided at either end of the waveguide 41 and used for connection with other waveguides and the like.
FIG. 10 is a sectional view taken along line A--A of FIG. 9 wherein 71, 72, and 73 indicate transmission paths of a characteristic impedance Z determined by the dimensions, i.e. the width and height of the waveguide 41, and FIGS. 11 and 12 show the equivalent circuits of the configuration shown in FIG. 10. Specifically, FIG. 11 shows the equivalent circuit under the ideal conditions that the thickness of the inductor plates 42, 43, 46, and 47 is zero. However, because there is really no case where the thickness of the inductor plates 42, 43, 46, and 47 is zero, there results in the equivalent circuit shown in FIG. 12. That is, as shown in FIG. 12, the thicknesses of the inductor plates 42, 43, 46, and 47 correspond in terms of circuitry to respective coils 74 through 77 inserted in series with the transmission path, and these coils 74 through 77 function so as to lower the center frequency of the pass band of the waveguide filter. Therefore, the thickness of the inductor plates, 42 through 49, being used in the waveguide filter must be as thin as possible. Reference numerals 78 and 79 indicate the equivalent elements of the inductor plates 42, 43, 46, and 47.
FIG. 13 is a fragmentary enlarged view of the slots 53 through 60 in which the inductor plates 42 through 49 are inserted. As shown in this drawing, because it is difficult in view of the machining technique to make strictly rectangular the point portion of the inductor plate 42, 46 as well as the bottom portion of the slot 53, 57, the point of the inductor plate 42, 46 would be cut obliquely and/or the bottom of the slot 53, 57 would be rounded in many cases.
FIG. 14 is a sectional view taken along line B--B of FIG. 9 and shows the state wherein the inductor plates 42 and 49 are attached to the waveguide 41. Because the inductor plate 42, 46 is thin, it is impossible to push the inductor plate strongly into the slot 53, 57 of the waveguide 41 and gaps 80 through 82 tend to appear between the point portions of the inductor plates 42 and 46 and the bottom portions of the slots 53 and 57 of the waveguide 41.
FIG. 15 is a sectional view taken along line C--C of FIG. 9 and shows the state wherein the inductor plates 42 through 49 are attached to the waveguide 41, like FIG. 14. Because the width of the slot 57, 58 does not coincide with the thickness of the inductor plate 46, 47, gaps 83 through 85 tend to appear between the inductor plates 46 and 47 and the waveguide 41.
FIG. 16 shows the state wherein the inductor plate 42 is secured to the waveguide 41 through soldering. In many cases, solder would flow into the interior of the waveguide 41 to create a convex portion 86 on the inner surface of the waveguide 41, or solder would flow defectively so as not to reach the inner surface of the waveguide 41 and a concave portion 87 would be created in the inner surface of the waveguide 41.
FIG. 17 is a perspective view of a general adjusting circuit for adjusting the center frequency of the waveguide circuit.
In FIG. 17, 88 indicates a waveguide of rectangular in cross section, having a metal screw 89 positioned at the center in the longitudinal direction and rectractable in the direction orthogonal to the longitudinal direction. 90 indicates a lock nut for fixing the metal screw 89.
According to the conventional waveguide filter of the foregoing configuration, there arise easily the gaps, convex portions, concave portions, etc. owing to the machining technique and the like, and these defects exert influence on the dimensional error and the surface current path of the waveguide filter, thus become the causes of deviation of the center frequency and the passband width of the waveguide filter.
Describing about the dimensional error, for example, in the case of a three-stage waveguide filter having the parameters: the center frequency=12 GHz, the passband width=200 MHz, the width of the waveguide=19.05 mm, its height=9.25 mm, and the longitudinal distance between the inductor plates=16.3-17.0 mm, from an error of 0.1 mm in the longitudinal distance between the inductor plates there results a variation of about 50 MHz in the center frequency, and from an error of 0.1 mm in the dimension of the inductor plate confined inside the waveguide and the width ofthe inductor window there results a change of about 12 MHz in the passband width.
Further, in the case of a three-stage waveguide filter having the parameters: the center frequency=50 GHz, the passband width=200 MHz, the width of the waveguide=4.78 mm, its height=2.39 mm, and the longitudinal distance between the inductor plates=3.6-3.7 mm, from an error of 0.01 mm in the longitudinal distance between the inductor plates there results a variation of about 90 MHz in the center frequency, and from an error of 0.01 mm in the dimension of the inductor plate confined inside the waveguide and in the width of the induction window there results a variation of about 10 MHz in the passband width.
As mentioned above, there was the drawback that even a slight error of dimension causes large variation of the center frequency and the passband width.
Some concrete causes of the above drawbacks are as follows: The point of the inductor plate is inevitably cut obliquely in view of the machining technique as shown in FIG. 13. The bottom of the slot in which the inductor plate is inserted is rounded in view of the machining technique also. Consequently, when the inductor plate is inserted into the slot there appear the gaps between the waveguide and the inductor plate as shown in FIG. 14, and it is impossible to take away these gaps because the inductor plate is thin and can not be pushed strongly at the time of assembly. Owing to these gaps, the dimensions of the inductor plate confined inside the waveguide and the width of the industion window vary and the passband width tends to deviate from a desired value.
Further, because it is impossible to make the thickness of the inductor plate coincide with the width of the slot, the gaps appear as shown in FIG. 15 and these gaps contribute to the occurrence of an error in the longitudinal distance between the inductor plates to thereby cause a deviation in the center frequency. Owing to these gaps, a return loss arises and/or the ripple charcteristic of the pass band becomes worse.
With the formation of convex portions and concave portions through soldering as shown in FIG. 16, the surface current path becomes long due to these convex portions and concave portions and the center frequency deviates from a desired value. In addition, because joining surfaces between the waveguide and the inductor plates are not smooth due to these convex portions and concave portions of solder, the high-frequency resistance of such a surface portion increases and the Q of the circuit lowers, thereby increasing the transmission loss of the pass band.
Further although the deviation of the center frequency will be adjusted by the adjusting circuit shown in FIG. 17, there exists the drawback that the transmission loss is large because of the presence of a convex portion, i.e. the metal screw 89 in the current path.
Furthermore, because the soldering process is carried out to effect fixation and the waveguide is made in the form of a single body, the process of exchanging parts or amending the connected state thereof that should be performed in the case of the presence of dimensional errors or some defects of the inductor plates or the waveguide becomes complicated, and this type of configuration is not suited for mass-production.